1. Field of the Invention
The present invention relates to estimating location of a communications device. In particular, the present invention relates to estimating location of a communications device based on signals transmitted between the communications device and antennas of a communication system.
2. Description of the Related Art
A communication system can be seen as a facility that enables communication between two or more entities such as user equipment and/or other nodes associated with the system. The communication may comprise, for example, communication of voice, data, multimedia and so on. The communication system may be circuit switched or packet switched. The communication system may be configured to provide wireless communication.
Positioning services have become very popular in the recent years. Positioning refers here to determining or estimating the location of a communications device. The communications device may be capable of determining its position based on signals it receives. The signals can be sent either from a specific positioning system or, for example, from a cellular communications system. Alternatively, the communications device may act as a measurement device and send measurement results to a further unit, which then determines the location of the communications device.
Positioning services may be used simply for locating the communications device. The location of the communications device may, for example, be shown on a map at the display of the communications device. Alternatively, it is possible to provide location-dependent services, for example, for users of a communication system. The location of the communications device may affect the content of a location-dependent service. A further option is that the location of the communications device is used for determining whether the service is provided to the communications device at all.
One important application for positioning services is emergency calls. It can be very useful to automatically determine the location of a communications device, from which an emergency call is originating. Certain countries have specified criteria for the accuracy of positioning a communications device. It is, however, appreciated that accuracy of the location estimates is important also for other applications than for emergency calls.
Locating a communications device in a communications system is typically based on measuring time differences between signals sent from antennas of the communications system and signals received from the communications device in the antennas of the communications system. Typically the accuracy of location estimation is increased by the number of antennas via which signals are transmitted between the communications device and the communications system.
FIG. 1a shows schematically one example, where the accuracy of location estimation may be quite low. In FIG. 1a, signals are transmitted between a communications device 101 and only one antenna 111 of a communications system. An antenna may be omni-directional or directional. Antenna profile refers to the angular distribution of radiation emitted from an antenna. The antenna 111 in FIG. 1a is directional, and the arrow in FIG. 1a shows schematically the direction into which radiation is emitted most efficiently from the antenna 111.
From measuring time differences between transmitted and received signals, it is possible to estimate the distance between the communications device 101 and the antenna 111. This distance, together with the antenna profile, defines an arc which represents the location estimate of the communications device. The angular width of the arc may be, for example, the half power beam width (HPBW) of the antenna. The width of the arc in a situation shown in FIG. 1a is typically quite wide, so the accuracy of the location estimate is quite poor.
It is appreciated that a similar situation than in FIG. 1a occurs, if signals are transmitted between a communications device and two or more antennas, which are located at the same antenna site. In this case, the measurements relating to the different antennas increase the accuracy of the distance measurement. Regarding the angular location, a common assumption is to locate the communications device at an angle where the antenna profiles of the antennas overlap. Based on the timing measurements, however, it is not possible to increase the accuracy of the angular location estimate.
FIG. 1b shows a second example, where the accuracy of location estimation may be quite low. In FIG. 1b, signals are transmitted between the communications device 101 and two antennas 111a, 111b of the communications system. These antennas 111a and 111b are located at separate antenna sites. Based on timing measurements, it is possible estimate the distance between the first antenna 111a and the communications device 101 and the distance between the second antenna 111b and the communications device 101. As FIG. 1b shows, these distance may define two arcs (circles) which cross each other at two points. The communications device is located near one of these points. Based on the timing measurements it is not possible to determine, which of the location estimates is a better location estimate for the communications device.
Timing measurements are typically available only for active radio links. To limit load in a communications system, there typically is a need to limit the number of active radio links allocated to a communications device. This limitation of active radio links makes the likelihood of calculating a location estimate based on information relating to only one antenna site or one antenna much higher than the likelihood of calculating a location estimate using measurement relating to two or more sites.
There are thus problems relating to estimating location of a communications device. Problems occur especially in cases, where the number of antenna sites involved in the location estimation is only one or two. It is, however, appreciated that similar problems may occur also when a larger number of antenna sites is involved in the location estimation.
Aim of embodiments of the present invention is to address the location estimation accuracy problems discussed above and to estimate location of a communications device.